The present invention relates to a method and apparatus for determining motor faults in an electric motor. More particularly, in an exemplary embodiment, the present invention relates to a method and apparatus for determining motor faults in an electric assist motor of an electric assist steering system.
Electric assist steering systems are known in the art. In electric assist steering systems, an electric assist motor, when energized, provides torque assist to aid the driver in turning steerable wheels of the vehicle. The electric assist motor is typically controlled in response to steering torque applied to the vehicle steering wheel and measured vehicle speed. A controller monitors applied steering torque and vehicle speed and controls a drive circuit to control electric current applied to the electric assist motor. A drive circuit in an electric assist steering system may include field effect transistors (xe2x80x9cFETsxe2x80x9d) or other forms of solid state switching devices operatively coupled between the vehicle battery and the electric assist motor. Motor current may be controlled by pulse-width-modulation (xe2x80x9cPWMxe2x80x9d) of the FETs or switches.
An electric assist steering system can use a permanent magnet alternating current (PMAC) motor. One particular type of PMAC motor is a three-phase PMAC motor. In a three-phase PMAC motor, the FETs are connected to provide three-phase voltage to the phases (A, B, C) of the motor. In operation of the three-phase PMAC motor, the A, B, and C phases of the motor are maintained 120 degrees (electrical) apart. For example, if phase A is at xcex8 degrees, then phase B would be at (xcex8+120) degrees, and phase C would be at (xcex8xe2x88x92120) degrees.
In operation, the amount of torque produced by the PMAC motor is functionally related to the amplitude of the electric current in the phases of the motor. The frequency of the electric current in the phases of the PMAC motor is selected to create a magnetic field in the phase windings that rotates about the armature at a predetermined speed. The rotating magnetic field induces the rotor to rotate. The rotational speed of the rotor is thus determined by the frequency of the electric current in the phases of the PMAC motor.
In operation, the rotating magnetic field may be commanded to lead the rotor by some angle. The angle between the rotating magnetic field and the rotor is referred to as an advance angle. The advance angle of the electric current in the phases of the PMAC motor can be controlled by adjusting the phase angle of the current supplied to the windings. Typically, when controlling a PMAC motor, the advance angle is increased as the rotor speed increases depending on motor torque and power requirements.
During operation, a motor may experience what are commonly referred to as xe2x80x9cmotor faultsxe2x80x9d. Motor faults may occur as a result of a variety of physical or structural failures of the motor. For example, in a PMAC motor, motor faults may occur when there is a short in a motor phase, when there is a phase-to-phase short in the motor phases, or when there is an open in a motor phase. Motor faults may cause the electric current realized in the motor phases to be different than the current commanded to the phases. As a result, the motor may not achieve the desired rotational speed or provide the desired torque.
In accordance with the present invention, a method for determining a motor fault in an electric motor includes the step of determining a calculated rotor position of the electric motor. Next, the calculated rotor position is compared to a measured rotor position of the electric motor to determine a rotor position error. Finally, a motor fault in the electric motor is determined when the rotor position error exceeds a predetermined maximum rotor position error.
Also, in accordance with the present invention, an apparatus for determining a motor fault in an electric motor determines a calculated rotor position of the electric motor. The apparatus then compares the calculated rotor position to a measured rotor position of the electric motor to determine a rotor position error. The apparatus determines a motor fault in the electric motor when the rotor position error exceeds a predetermined maximum rotor position error.